Condenser microphones have various applications. For example, some condenser microphones are installed on a speech table in a conference room or a church. These condenser microphones comprise a proximity sensor that turns on a microphone output only when there is a speaker standing in front of the speech table; the proximity sensor turns off the microphone output when there is no speaker standing in front of the speech table.
Proximity sensors are classified into a supersonic type, an electrostatic type, an infrared type, and the like. For microphones, the infrared proximity sensor is generally used, which consists of an infrared light emitting diode not serving as a noise source and an infrared photodetector.
However, the infrared proximity sensor may malfunction when the speaker is at a standstill or owing to external light, a variation in temperature, or the like. Thus, to improve the reliability of the proximity sensor, an oscillation circuit is used to light an infrared light emitting diode at a predetermined frequency using an AC component. Further, an infrared photodetector tuned to this frequency is used in combination with the infrared light emitting diode.
In this case, an infrared beam is radiated to the speaker. Then, the infrared beam reflected by the speaker is received. Accordingly, the infrared light emitting diode needs to have a high radiant intensity. The infrared photodetector needs to have a high sensitivity. To increase the radiant intensity of the infrared light emitting diode, a forward current passed through the diode must be increased. For example, for infrared light emitting diodes TLN119 manufactured by TOSHIBA CORPORATION, a current of about 10 mA is required.
In this regard, a phantom power source generally used for condenser microphones provides an excessively high voltage. Accordingly, a down converter must be used to reduce the voltage to increase the current. The down converter is based on one of various methods; a pulse width modulation type is often used.
However, the pulse width modulation down converter interrupts an input voltage. Consequently, the pulse width modulation down converter makes noise of a very high level in connection with the switching. The noise may be mixed into the power supply voltage to increase the amount of noise from the microphone.
Further, the down converter is provided with an inductor. Accordingly, a noisy current flowing through the inductor results in an external magnetic field, which may be magnetically coupled to an output transformer in the microphone. As a result, noise may be output.
On the other hand, only a very small amount of infrared beam is reflected by the speaker. Accordingly, a very sensitive infrared photodetector is used. However, when the AC component used to light the infrared light emitting diode is superimposed on the power supply to the infrared photodetector, the infrared photodetector malfunctions as if it received an infrared beam, because of its high sensitivity.